1. Field of the Invention
The present invention relates to pulse doppler radar; and more particularly, to an improved method and system for tracking targets.
2. Description of the Prior Art
In tracking moving targets, a pulse doppler radar system typically directs its antenna or other control system, if applicable, toward an echo signal that exhibits a particular doppler frequency shift and amplitude. Such systems exhibit a high degree of reliability in that, among other things, they are capable of rejecting undesirable signals that are caused by background clutter, for example. In discriminating between signals that represent a true target and signals that represent background clutter, the system typically uses a notch or highpass filter that rejects signals having a doppler frequency shift that correspond to main beam background clutter or other undesirable signals. However, under certain circumstances, the system may experience a blind region which occurs whenever the doppler frequency shift of the target coincides with the doppler frequency of the background main beam clutter. Under such circumstances, both the true target and the clutter frequency is rejected by the filter. The probability of such coincidence, and thus loss of the target, may be lessened by providing a filter that "notches out" or rejects a narrower frequency spectrum. However, such a solution, although satisfactory in many instances, results in a loss of capability of detecting desirable target signals under certain circumstances. Nevertheless, even under such circumstances, where the tracker momentarily loses its target, the system is provided with an estimating means such as a Kalman filter. With such an estimating means, the last known target position prior to the entry of the target in the blind region is extrapolated using available estimates of velocity and acceleration. During this estimating mode, the radar return signals are continuously monitored so that tracking may resume, if and when, the target's return signals emerge from the clutter notch rejection band; provided that the target's position in space is sufficiently close to the extrapolated position to permit radar illumination, detection, and tracking reacquisition. Although apparatus, which provides such a method of target reacquisition after tracking through a blind region, has proven effective under certain conditions, failure to reacquire the target may occur under conditions where the target fails to emerge from the clutter notch within a certain period of time. Also, the target may be able to defeat such reacqusition by performing high speed acceleration reversal maneuvers. Even when target reacquisition is successful after the system has completed the estimation mode, considerable transients may be generated by such reacquisition because extrapolation is seldom perfect due to radar noise and target maneuvers. Such transients are usually visible to the pilot which tends to reduce his confidence in the system.
Thus, it would be desirable to provide a method and system that would be able to continue the tracking of a target even though the doppler frequency of the target is rejected by the clutter notch filter of the system.